Ok so I was in school, and I had FireRed or LeafGreen. I had a friend who wanted to trade with me. He promised he'd give me shinies and nice stuff, and I guess we couldn't trade in school or something so I ended up actually giving him my cartridge like the idiot that I was.
The next day he gives me my cartridge back and when I turned it on I discovered that he took almost all of my good Pokemon and left me with almost nothing. I was super pissed at him for a while, but this motivated me to spend extra time on the game so I ended up raising a level 100 Tyranitar and other powerful Pokemon, and then competed and won lots of battles in the Trainer Tower. This was my best memory. My worst is when I found shinies that I couldn't catch! >_>